Muscle triacylglycerol and hormone-sensitive lipase activity in untrained and trained human muscles

Relationship between low relative muscle mass and aortic regional morphological changes in adults underwent contrast CT scans for cancer diagnostics

OBJECTIVES

Low muscle mass is related to cardiovascular risk factors. This study aimed to investigate whether relative low muscle mass is related to the diameter and tortuosity of the aorta.

METHODS

We performed a cross-sectional study of 208 adults without known cardiovascular disease who underwent Computed Tomography (CT) enhanced scan between 2020 and 2021. Skeletal muscle index (SMI) was estimated. The morphology of the aorta was measured by diameter and tortuosity using CT. We assessed the relationship between SMI and diameter and tortuosity of the aorta using Spearman correlation analysis and univariate and multivariate-adjusted regression models.

RESULTS

Of all -individuals, 124 (59.6%) were male. The average age was 60.13 ± 16.33 years old. SMI was inversely associated with the diameter and tortuosity of the aorta (p < 0.05). Specifically, in a multivariable-adjusted model adjusting for potential confounders, a one-unit increase in the SMI was associated with a -13.56mm(95% confidence intervals (CI): -18.16 to -8.96, p < 0.001), -7.93 mm (95% CI: -10.85 to -5.02, p < 0.001), -8.01 mm (95% CI: -11.30 to -4.73, p < 0.001), -5.16 mm (95% CI: -7.57 to -2.75, p < 0.001) and -2.73 mm (95% CI: -5.18 to -0.27, p = 0.031) increase in L1-L5 diameter respectively, a -0.89 (95% CI: -1.14 to -0.64, p < 0.001) increase in the aorta tortuosity, a -0.48 (95% CI: -0.59 to -0.36, p < 0.001) increase in the descending thoracic aorta tortuosity, and a -0.44 (95% CI: -0.52 to -0.35, p < 0.001) increase in the abdominal aorta tortuosity.

CONCLUSIONS

Relative muscle mass was negatively associated with the diameter and tortuosity of the aorta, suggesting muscle mass maintenance may play a role in preventing aortic morphological changes.

The Effects of Blood Flow Restricted Electrostimulation on Strength and Hypertrophy

Context: The combined effect of neuromuscular electrical stimulation (NMES) and blood flow restriction (BFR) on muscle mass and strength has not been thoroughly investigated. Objective: To examine the effects of combined and independent BFR and a low-intensity NMES on skeletal muscle adaptation. Design: Exploratory study. Setting: Laboratory. Participants: Twenty recreationally active subjects. Main Outcome Measures: Subjects had each leg randomly allocated to 1 of 4 possible intervention groups: (1) cyclic BFR alone, (2) NMES alone, (3) BFR + NMES, or (4) control. Each leg was stimulated in its respective intervention group for 32 minutes, 4 days per week for 6 weeks. Mean differences in size (in grams) and isometric strength (in kilograms), between week 0 and week 6, were calculated for each group. Results: Leg strength increased 32 (19) kg in the BFR + NMES group, which differed from the 3 (11) kg change in the control group (P = .03). The isolated NMES and BFR groups revealed increases of 16 (28) kg and 18 (17) kg, respectively, but these did not statistically differ from the control, or one another. No alterations were statistically significant for leg size. Conclusion: Compared with a control that received no treatment, the novel combination of BFR and NMES led to increasing muscular strength of the knee extensors, but not muscle mass which had a large interindividual variability in response.

Triglyceride metabolism in exercising muscle

Triglycerides are stored within lipid droplets in skeletal muscle and can be hydrolyzed to produce fatty acids for energy production through β-oxidation and oxidative phosphorylation. While there was some controversy regarding the quantitative importance of intramyocellular triglyceride (IMTG) as a metabolic substrate, recent advances in proton magnetic resonance spectroscopy and confocal microscopy support earlier tracer and biopsy studies demonstrating a substantial contribution of IMTG to energy production, particularly during moderate-intensity endurance exercise. This review provides an update on the understanding of IMTG utilization during exercise, with a focus on describing the key regulatory proteins that control IMTG breakdown and how these proteins respond to acute exercise and in the adaptation to exercise training. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Muscle growth across a variety of exercise modalities and intensities: Contributions of mechanical and metabolic stimuli

This paper reviews the existing evidence for the potential contribution of metabolic and mechanical stimuli to muscle growth in response to a variety of exercise modalities and intensities. Recent research has demonstrated that low-load resistance training can elicit comparable hypertrophy to that of high-load resistance training when each set is performed until failure. The degree of metabolic fatigue would be greater for resistance training with lower loads compared to higher loads at the point of muscle failure, which may compensate for the lower mechanical stress. This may also explain why muscle hypertrophy occurs to varying magnitudes when activities such as cycling and walking are performed. Furthermore, the application of blood flow restriction to the working muscles during these activities induces greater hypertrophy albeit at the same level of mechanical stress, which would suggest a possible contribution from metabolic stress. Thus, it is plausible that both mechanical and metabolic stimuli are primary mechanisms for muscle hypertrophy and the degree of contributions of both stimuli determines the exercise-induced muscle hypertrophy.

Exercise and Regulation of Lipid Metabolism

The increased prevalence of hyperlipidemia, hypertriglyceridemia, hypercholesterolemia, and fatty liver disease has provided increasingly negative connotations toward lipids. However, it is important to remember that lipids are essential components supporting life. Lipids are a class of molecules defined by their inherent insolubility in water. In biological systems, lipids are either hydrophobic (containing only polar groups) or amphipathic (possess polar and nonpolar groups). These characteristics lend lipids to be highly diverse with a multitude of functions including hormone and membrane synthesis, involvement in numerous signaling cascades, as well as serving as a source of metabolic fuel supporting energy production. Exercise can induce changes in the lipid composition of membranes that effect fluidity and cellular function, as well as modify the cellular and circulating environment of lipids that regulate signaling cascades. The purpose of this chapter is to focus on lipid utilization as metabolic fuel in response to acute and chronic exercise training. Lipids utilized as an energy source during exercise include circulating fatty acids bound to albumin, triglycerides stored in very-low-density lipoprotein, and intramuscular triglyceride stores. Dynamic changes in these lipid pools during and after exercise are discussed, as well as key factors that may be responsible for regulating changes in fat oxidation in response to varying exercise conditions.

Adipose triglyceride lipase deletion from adipocytes, but not skeletal myocytes, impairs acute exercise performance in mice

Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme mediating triacylglycerol hydrolysis in virtually all cells, including adipocytes and skeletal myocytes, and hence, plays a critical role in mobilizing fatty acids. Global ATGL deficiency promotes skeletal myopathy and exercise intolerance in mice and humans, and yet the tissue-specific contributions to these phenotypes remain unknown. The goal of this study was to determine the relative contribution of ATGL-mediated triacylglycerol hydrolysis in adipocytes vs. skeletal myocytes to acute exercise performance. To achieve this goal, we generated murine models with adipocyte- and skeletal myocyte-specific targeted deletion of ATGL. We then subjected untrained mice to acute peak and submaximal exercise interventions and assessed exercise performance and energy substrate metabolism. Impaired ATGL-mediated lipolysis within adipocytes reduced peak and submaximal exercise performance, reduced peripheral energy substrate availability, shifted energy substrate preference toward carbohydrate oxidation, and decreased HSL Ser(660) phosphorylation and mitochondrial respiration within skeletal muscle. In contrast, impaired ATGL-mediated lipolysis within skeletal myocytes was not sufficient to reduce peak and submaximal exercise performance or peripheral energy substrate availability and instead tended to enhance metabolic flexibility during peak exercise. Furthermore, the expanded intramyocellular triacylglycerol pool in these mice was reduced following exercise in association with preserved HSL phosphorylation, suggesting that HSL may compensate for impaired ATGL action in skeletal muscle during exercise. These data suggest that adipocyte rather than skeletal myocyte ATGL-mediated lipolysis plays a greater role during acute exercise in part because of compensatory mechanisms that maintain lipolysis in muscle, but not adipose tissue, when ATGL is absent.

Relationship between thigh muscle mass and augmented pressure from wave reflections in healthy adults

Skeletal muscle may be viewed as an endocrine organ that releases numerous factors with the potential to influence vascular tone. Previous cross-sectional studies have shown an inverse relationship between muscle mass and arterial stiffness. We examined the relationship between muscle mass, arterial pressure in the aorta and brachial artery, and pressure from wave reflections [characterized as heart rate corrected augmentation pressure (AP)] and augmentation index (AIx). Twenty-seven (13 male, 14 female) subjects who were non-smokers and had no known cardiovascular or metabolic diseases visited the laboratory for two sessions of testing. Upon arriving for the first session, mid-thigh muscle (mCSA) and fat (fCSA) cross-sectional area were assessed using peripheral Quantitative Computed Tomography. Following this, concentric one-repetition maximum (1-RM) testing was completed to assess knee extensor strength. The second visit consisted of taking brachial and aortic blood pressure measurements. A significant positive relationship was found between mCSA and brachial systolic blood pressure (r = 0.47, p = 0.02), but not between mCSA and aortic systolic blood pressure (r = 0.35, p = 0.09). There was an inverse association between mCSA and AP75 (-0.49, p = 0.01) and AIx75 (-0.49, p = 0.01). In conclusion, muscle mass is associated with brachial systolic blood pressure and inversely associated with pressure from wave reflections. Our findings suggest a link between global musculo-skeletal integrity and cardiovascular hemodynamics in young healthy adults.

Overexpression of PGC-1α increases fatty acid oxidative capacity of human skeletal muscle cells

We investigated the effects of PGC-1α (peroxisome proliferator-activated receptor γ coactivator-1α) overexpression on the oxidative capacity of human skeletal muscle cells ex vivo. PGC-1α overexpression increased the oxidation rate of palmitic acid and mRNA expression of genes regulating lipid metabolism, mitochondrial biogenesis, and function in human myotubes. Basal and insulin-stimulated deoxyglucose uptake were decreased, possibly due to upregulation of PDK4 mRNA. Expression of fast fiber-type gene marker (MHCIIa) was decreased. Compared to skeletal muscle in vivo, PGC-1α overexpression increased expression of several genes, which were downregulated during the process of cell isolation and culturing. In conclusion, PGC-1α overexpression increased oxidative capacity of cultured myotubes by improving lipid metabolism, increasing expression of genes involved in regulation of mitochondrial function and biogenesis, and decreasing expression of MHCIIa. These results suggest that therapies aimed at increasing PGC-1α expression may have utility in treatment of obesity and obesity-related diseases.

Fat oxidation, fitness and skeletal muscle expression of oxidative/lipid metabolism genes in South Asians: implications for insulin resistance?

Background

South Asians are more insulin resistant than Europeans, which cannot be fully explained by differences in adiposity. We investigated whether differences in oxidative capacity and capacity for fatty acid utilisation in South Asians might contribute, using a range of whole-body and skeletal muscle measures.

Methodology/Principal Findings

Twenty men of South Asian ethnic origin and 20 age and BMI-matched men of white European descent underwent exercise and metabolic testing and provided a muscle biopsy to determine expression of oxidative and lipid metabolism genes and of insulin signalling proteins. In analyses adjusted for age, BMI, fat mass and physical activity, South Asians, compared to Europeans, exhibited; reduced insulin sensitivity by 26% (p = 0.010); lower VO_2max (40.6±6.6 vs 52.4±5.7 ml.kg⁻¹.min⁻¹, p = 0.001); and reduced fat oxidation during submaximal exercise at the same relative (3.77±2.02 vs 6.55±2.60 mg.kg⁻¹.min⁻¹ at 55% VO_2max, p = 0.013), and absolute (3.46±2.20 vs 6.00±1.93 mg.kg⁻¹.min⁻¹ at 25 ml O₂.kg⁻¹.min⁻¹, p = 0.021), exercise intensities. South Asians exhibited significantly higher skeletal muscle gene expression of CPT1A and FASN and significantly lower skeletal muscle protein expression of PI3K and PKB Ser473 phosphorylation. Fat oxidation during submaximal exercise and VO_2max both correlated significantly with insulin sensitivity index and PKB Ser473 phosphorylation, with VO_2max or fat oxidation during exercise explaining 10–13% of the variance in insulin sensitivity index, independent of age, body composition and physical activity.

Conclusions/Significance

These data indicate that reduced oxidative capacity and capacity for fatty acid utilisation at the whole body level are key features of the insulin resistant phenotype observed in South Asians, but that this is not the consequence of reduced skeletal muscle expression of oxidative and lipid metabolism genes.

The effect of exercise and nutrition on intramuscular fat metabolism and insulin sensitivity

Intramuscular triacylglycerol (IMTG) is both a dynamic fat-storage depot that can expand during periods of elevated lipid availability and a fatty acid source that can be utilized during periods of increased energy expenditure in active individuals. Although many studies have investigated the lifestyle determinants of IMTG content, the results are far from consistent, and studies attempting to unravel the mechanisms behind IMTG metabolism are in their infancy. The limited evidence available suggests that the enzymes responsible for skeletal muscle lipolysis and IMTG synthesis play an important role in determining the fate of fatty acids and therefore the concentration of lipid metabolites and insulin sensitivity of skeletal muscle. This review provides a summary of current knowledge on the effects of acute and chronic exercise as well as energy intake and macronutrient composition of the diet upon the metabolism of IMTG and the implications for metabolic health.

Are substrate use during exercise and mitochondrial respiratory capacity decreased in arm and leg muscle in type 2 diabetes?

AIM/HYPOTHESIS

The aim of the study was to investigate mitochondrial function, fibre type distribution and substrate oxidation in arm and leg muscle during exercise in patients with type 2 diabetes and in obese and lean controls.

METHODS

Indirect calorimetry was used to calculate fat and carbohydrate oxidation during both progressive arm-cranking and leg-cycling exercises. Muscle biopsies from arm and leg were obtained. Fibre type, as well as O(2) flux capacity of saponin-permeabilised muscle fibres were measured, the latter by high resolution respirometry, in patients with type 2 diabetes, age- and BMI-matched obese controls, and age-matched lean controls.

RESULTS

Fat oxidation was similar in the groups during either arm or leg exercise. During leg exercise at higher intensities, but not during arm exercise, carbohydrate oxidation was lower in patients with type 2 diabetes compared with the other groups. In patients with type 2 diabetes, ADP-stimulated state 3 respiration per mg muscle with parallel electron input from complex I+II was lower in m. vastus lateralis compared with obese and lean controls, whereas no differences between groups were present in m. deltoideus. A higher percentage of type IIX fibres was seen in m. vastus lateralis in patients with type 2 diabetes compared with obese and lean controls, whereas no difference was found in the deltoid muscle.

CONCLUSIONS/INTERPRETATION

This study demonstrates similar O(2) flux capacity, fibre type distribution and carbohydrate oxidation in arm muscle in the groups despite the presence of attenuated values in leg muscle in patients with type 2 diabetes compared with obese and lean controls.

Adipose triglyceride lipase in human skeletal muscle is upregulated by exercise training

Mobilization of fatty acids from stored triacylglycerol (TG) in adipose tissue and skeletal muscle [intramyocellular triacylglycerol (IMTG)] requires activity of lipases. Although exercise training increases the lipolytic capacity of skeletal muscle, the expression of hormone-sensitive lipase (HSL) is not changed. Recently, adipose triglyceride lipase (ATGL) was identified as a TG-specific lipase in various rodent tissues. To investigate whether human skeletal muscle ATGL protein is regulated by endurance exercise training, 10 healthy young men completed 8 wk of supervised endurance exercise training. Western blotting analysis on lysates of skeletal muscle biopsy samples revealed that exercise training induced a twofold increase in skeletal muscle ATGL protein content. In contrast to ATGL, expression of comparative gene identification 58 (CGI-58), the activating protein of ATGL, and HSL protein was not significantly changed after the training period. The IMTG concentration was significantly decreased by 28% at termination of the training program compared with before. HSL-phoshorylation at Ser(660) was increased, HSL-Ser(659) phosporylation was unchanged, and HSL-phoshorylation at Ser(565) was decreased altogether, indicating an enhanced basal activity of this lipase. No change was found in the expression of diacylglycerol acyl transferase 1 (DGAT1) after training. Inhibition of HSL with a monospecific, small molecule inhibitor (76-0079) and stimulation of ATGL with CGI-58 revealed that significant ATGL activity is present in human skeletal muscle. These results suggest that ATGL in addition to HSL may be important for human skeletal muscle lipolysis.

Human skeletal muscle ceramide content is not a major factor in muscle insulin sensitivity

AIMS/HYPOTHESIS

In skeletal muscle, ceramides may be involved in the pathogenesis of insulin resistance through an attenuation of insulin signalling. This study investigated total skeletal muscle ceramide fatty acid content in participants exhibiting a wide range of insulin sensitivities.

METHODS

The middle-aged male participants (n=33) were matched for lean body mass and divided into four groups: type 2 diabetes (T2D, n=8), impaired glucose tolerance (IGT, n=9), healthy controls (CON, n=8) and endurance-trained (TR, n=8). A two step (28 and 80 mU m(-2) min(-1)) sequential euglycaemic-hyperinsulinaemic clamp was performed for 120 and 90 min for step 1 and step 2, respectively. Muscle biopsies were obtained from vastus lateralis at baseline, and after steps 1 and 2.

RESULTS

Glucose infusion rates increased in response to insulin infusion, and significant differences were present between groups (T2D<IGT<CON<TR). At baseline, muscle ceramide content was 108+/-7, 95+/-6, 126+/-12 and 156+/-25 nmol total ceramide fatty acids/g wet weight of tissue in the T2D, IGT, CON and TR groups, respectively, and muscle ceramide content was higher (p<0.01) in the TR than the IGT group. Muscle ceramide content was not influenced by insulin infusion. Interestingly, a positive correlation (r=0.42, p<0.05) was present between muscle ceramide content at baseline and insulin sensitivity.

CONCLUSIONS/INTERPRETATION

Total muscle ceramide content was similar between individuals showing marked differences in insulin sensitivity, and therefore does not seem to be a major factor in muscle insulin resistance. Furthermore, aerobic capacity does not appear to influence muscle ceramide content.

Determinants of intramyocellular triglyceride turnover: implications for insulin sensitivity

Increased intramyocellular triglyceride (IMTG) content is found in both insulin-sensitive endurance-trained subjects and insulin-resistant obese/type 2 diabetic subjects. A high turnover rate of the IMTG pool in athletes is proposed to reduce accumulation of lipotoxic intermediates interfering with insulin signaling. IMTG turnover is a composite measure of the dynamic balance between lipolysis and lipid synthesis; both are influenced by mitochondrial fat oxidation and plasma free fatty acid availability. Therefore, more attention should be given to the factors controlling the rate of turnover of IMTG. In this review, particular attention has been given to muscle oxidative capacity, plasma free fatty acid availability, and IMTG hydrolysis (lipolysis) and synthesis. A higher oxidative, lipolytic, and lipid storage capacity in the muscle of endurance-trained subjects reflects a higher fractional turnover of the IMTG pool. Thus the co-localization of intermyofibrillar lipid droplets and mitochondria allows for a fine coupling of lipolysis of the IMTG pool to mitochondrial beta-oxidation. Conversely, reduced oxidative capacity and a mismatch between IMTG lipolysis and beta-oxidation might be detrimental to insulin sensitivity by generating several lipotoxic intermediates in sedentary populations including obese/type 2 diabetic subjects. Further studies are clearly required to better understand the relationship between the rate of turnover of IMTG and the accumulation of lipotoxic intermediates in the pathophysiology of insulin resistance.

Resistance exercise: good for more than just Grandma and Grandpa’s muscles

Progressive resistance training promotes strength gains in both the young and the aged. Importantly, gains in strength in aged persons are, with the appropriate duration, intensity, and progression, not simply due to neuromuscular mechanisms, but also encompass muscle fibre hypertrophy. Critically, the resistance exercise-induced changes in aged skeletal muscle are associated with numerous health benefits, the most obvious of which are the gains in strength and, with the correct training program, power; as a result, functional independence is improved and the risk for falls is apparently reduced. Aside from the well-documented effects of resistance training on strength and power, a body of research is now beginning to emerge that shows resistance exercise also promotes metabolic health. This is crucial information, since it effectively highlights an underappreciated aspect of resistance exercise. Specifically, resistance exercise not only promotes strength gains, but also reduces risk for diabetes and cardiovascular disease. The benefits of resistance exercise do not end at metabolic health, however, and “spill over” into many other realms. In fact, resistance exercise programs have been shown to reduce participants’ use of the health care system. Viewed collectively, the multiple benefits of resistance exercise represent an attractive option for our aging population to enhance and maintain their health from a number of perspectives that are not achievable through pharmacological intervention or with solely aerobic-based exercise.

Resistance training, insulin sensitivity and muscle function in the elderly

Ageing is associated with a loss in both muscle mass and in the metabolic quality of skeletal muscle. This leads to sarcopenia and reduced daily function, as well as to an increased risk for development of insulin resistance and type 2 diabetes. A major part, but not all, of these changes are associated with an age-related decrease in the physical activity level and can be counteracted by increased physical activity of a resistive nature. Strength training has been shown to improve insulin-stimulated glucose uptake in both healthy elderly individuals and patients with manifest diabetes, and likewise to improve muscle strength in both elderly healthy individuals and in elderly individuals with chronic disease. The increased strength is coupled to improved function and a decreased risk for fall injuries and fractures. Elderly individuals have preserved the capacity to improve muscle strength and mass with training, but seem to display a reduced sensitivity towards stimulating protein synthesis from nutritional intake, rather than by any reduced response in protein turnover to exercise.